Implantable cardiac stimulation devices (such as pacemakers, defibrillators, and cardioverters) are designed to monitor and stimulate the heart of a patient that suffers from a cardiac arrhythmia. Using leads connected to a patient's heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to detected cardiac events which are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, the shape, the location, and the frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program (which is responsible for the operation of the implantable device) can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Modern implantable devices have a great number of adjustable parameters that must be tailored to a particular patient's therapeutic needs. One adjustable parameter of particular importance in stimulation devices is the stimulus energy (i.e., the pulse amplitude and pulse width) which can be programmed to new values in response to changes in capture threshold. "Capture" is defined as a cardiac depolarization and contraction of the heart in response to a stimulation pulse. When a stimulation pulse stimulates either a patient's atrium or ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a "capture threshold" which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should be achieved at the lowest possible energy setting yet provide enough of a safety margin so that should a patient's threshold increase, the output of an implantable stimulation device (i.e. the pacing stimulus energy) will still be sufficient to maintain capture. Dual chamber stimulation devices may have different atrial and ventricular pacing stimulus energies that correspond to different atrial and ventricular capture thresholds, respectively.
The earliest stimulation devices had a predetermined and unchangeable pacing stimulus energy, which proved to be problematic because the capture threshold is not a static value. The capture threshold also may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around stimulation device lead tips within several months after implantation may cause an increase in the capture threshold. To avoid loss of capture, the earliest stimulation devices were preset to deliver pacing pulses at the maximum energy available. As a result some patients experienced discomfort because of the high level of stimulation. Furthermore, such stimulation pulses consumed extra battery resources, thus shortening the useful life of a stimulation device.
When programmable stimulation devices were developed, the pacing stimulus energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted stimulation device via telemetry or via a magnet applied to a patient's chest. The particular setting for the stimulation device's pacing threshold was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient's capture threshold, from the patient's medical history, and from a listing of the patient's medications. This improvement in adjustable pacing stimulus energy permitted programming to lower values that tended to conserve battery energy and extend the useful service life of the stimulation device.
Also, patients who experienced discomfort due to excessively high stimulus energy pulses could have the stimulus energy safely decreased thus, lessening the incidence of surgical revision of the pacing system. While the adjustable pacing stimulus energy feature proved to be superior to the previously known static stimulus energy, some significant problems remained unsolved. In particular, when a patient's capture threshold changed, the patient was forced to visit the medical practitioner to adjust the pacing stimulus energy accordingly.
To address this pressing problem, manufacturers have developed advanced stimulation devices that are capable of determining a patient's capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This approach, referred to herein as "autocapture", improves the patient's comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the stimulation device's battery life by conserving the energy used for stimulation pulses.
A common technique used to determine whether capture has been effectuated is to monitor the patient's cardiac activity and to search for presence of an "evoked response" following a stimulation pulse. The evoked response is an electrical event that is the response of the heart to the application of a stimulation pulse thereto. The patient's heart activity is typically monitored by the stimulation device by keeping track of the stimulation pulses delivered to the heart and by examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue(myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by the generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by the generation of an R-wave (sometimes referred to as the "QRS" complex when viewed on an ECG strip).
When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the atrium (hereinafter referred to as an "A-pulse"), any response sensed by atrial sensing circuits of the stimulation device immediately following application of the A-pulse is presumed to be an evoked response that evidences capture of the atria.
However, it is for several reasons very difficult to detect a true atrial evoked response. First, a high energy A-pulse may obscure the evoked response signal, making it difficult to detect and identify. Second, the signal sensed by the atrial sensing circuitry immediately following the application of an A-pulse may be not an evoked response, but noise--either electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point where an electrode of the lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as the A-pulse, across the interface. Unfortunately, because the atrial evoked response is sensed through the same lead electrode through which the A-pulse is delivered, the resulting polarization signal formed at the electrode can corrupt the evoked response sensed by the atrial sensing circuits. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and other variables, many of which are continually changing over time.
In each case, the result may be a false positive detection of an atrial evoked response. Such an error leads to a false atrial capture indication, which in turn leads to missed heartbeats--a highly undesirable and potentially a life-threatening situation. Another problem results from a failure by the stimulation device to detect an atrial evoked response that has actually occurred. In this case, a loss of atrial capture is indicated when atrial capture is in fact present--also an undesirable situation that will cause the stimulation device to unnecessarily invoke the atrial pacing threshold determination function and result in higher than necessary stimulus energy values.
Because of the problems previously stated regarding the test for atrial capture verification and automatic threshold tests, currently available stimulation devices do not have this capability. As a result, many medical practitioners manually conduct atrial capture verification tests during periodic follow up examinations. These periodic follow-up examinations are performed by the medical practitioner after initial implantation and configuration of the stimulation device to determine whether the therapy delivered by the device is having the desired effect and to verify the proper operation. Capture verification and pacing threshold assessment is typically performed by the medical practitioner using an external programmer for controlling the stimulation device functions in conjunction with a surface electrocardiogram (ECG) device.
However, this common capture verification and pacing threshold assessment procedure is a time consuming and complex task requiring significant attention and effort on the part of the medical practitioner. The medical practitioner must spend a significant amount of time placing and subsequent removal of ECG electrodes, and configuring the ECG system for the patient's individual characteristics. The practitioner must also manually examine the ECG readout and analyze the cardiac waveform to determine whether capture is present both during initial capture verification and during the pacing threshold determination tests.
It would thus be desirable to provide a system and method for enabling the stimulation device to automatically perform atrial capture verification and atrial pacing threshold determination without a medical practitioner's involvement. It would also be desirable to enable the stimulation device to perform the atrial capture verification and atrial pacing threshold determination without requiring dedicated circuitry and/or special sensors. It would further be desirable to maintain a record of atrial pacing threshold determination in the stimulation device so that a medical practitioner can verify the proper operation of the stimulation device by examining the record.